Photooxidation Mechanism
of Methanol on Rutile TiO<sub>2</sub> Nanoparticles
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Abstract
The use of nanoparticulate TiO<sub>2</sub> as a photocatalyst
for
the conversion of organic molecules has grown tremendously in recent
years; however, the roles of excited electrons, holes, and surface
adsorbates in titania photochemistry remain poorly understood. In
this work, detailed infrared measurements, which are sensitive to
both vibrational and electronic transitions within the material, are
used to uncover the mechanism of methanol oxidation on 4 nm rutile
nanoparticles in both anaerobic and aerobic conditions. These experiments
are performed in an ultrahigh vacuum cell where the coverage of methanol
and exposure to oxygen are precisely controlled. Our measurements
reveal that the primary pathway for initial methanol adsorption on
TiO<sub>2</sub> is dissociative, leading to the production of adsorbed
methoxy groups. Upon exposure of the sample to ultraviolet photons,
the results show that the electron–hole pairs (e<sup>–</sup>–h<sup>+</sup>) generated within TiO<sub>2</sub> have significant
lifetimes because the holes are efficiently trapped by the surface
methoxy groups. The subsequent photochemistry induces a two-electron
oxidative degradation process of the surface methoxy groups to formate.
Formate production proceeds through the formation of a radical anion,
the result of hole oxidation, followed by prompt electron injection
by the radical anion into the TiO<sub>2</sub>. Furthermore, these
studies show that the role of O<sub>2</sub> in promoting methanol
photodecomposition is to scavenge free electrons, which opens acceptor
sites for the injection of new electrons during methoxy group oxidation.
In this way, O<sub>2</sub> increases the efficiency of methoxy oxidation
by a factor of 5 relative to anaerobic conditions, yet does not affect
the hole-mediated oxidation mechanism that leads to final formate
production